For PLD it has niche uses. Burtgang is just so dang good. Sakpata is also a nice sword for what it is.
For RDM it offers utility (refresh+3, and the Regen +20 gives you about 50 tic with Regen II), a pretty solid fusion/light based weaponskill, and added effect damage (which is good when a WHM is ruining your T1 Enspells.)
It's going to boost your white damage a bit in situations where you are stuck with Enspells IIs, or if magic damage is no bueno. The added effect deals slashing damage and can proc on multi hits of the main hand.
You can pair it with a Thib offhand to keep Savage Blade Damage boosted. If accuracy is an issue you can pair it with Gleti's/Crep (if you do not have a flame tongue.)
In a defensive set, it is also nice with a Sakpata Sword.
You might also want to use those daggers offhand to have higher Macc (while still dealing out decent damage) The skill on it is pretty decent but you are also losing Naegling's/Croc's raw Macc/attributes.
If it's a boss by all means swap weapons on cast and bring your A-Game.
Refresh+3 can be nice at times, and with a nice Regen set I was getting about 50 tic Regen. Doesn't seem like much but it enables you to focus more on DPS/Keeping Stoneskin and shadows up for certain fights.
The added effect is also nice for those times you have a WHM ruining your tier 1 enspells. Or if an enemy takes pretty lousy magic damage to begin with
I've been enjoying mine, it's another tool in our kit and has it's uses. Basically you are trading Naegling's raw damage/Macc for some utility, white damage and refresh/survival.
I recommend making Excalibur and Mandau if you love RDM. Mandau R15 is especially nice for Sheol C.
Temper II basically transforms relic weapons from something mediocre to something that is surprisingly good. With dual Wield it becomes a 3-6 hit weaponskill. If using a Crep knife it raises the max to 8 hits.
The first hit on Mercy stroke/KOTR is 8.4 fTP
Additional hits are 1.68 "fTP".
But hey I went off topic. Back to the PLD stuff.
Did I mention Excalibur is a sexy lock style?